September 15, 2010 / Vol. 35, No. 18 / OPTICS LETTERS
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Optical fiber microwire current sensor M. Belal,1,* Z. Song,1,2 Y. Jung,1 G. Brambilla,1 and T. P. Newson1 1 2
ORC, University of Southampton, Southampton, SO17 1BJ, UK
College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China *Corresponding author:
[email protected] Received June 23, 2010; revised August 2, 2010; accepted August 11, 2010; posted August 16, 2010 (Doc. ID 130608); published September 3, 2010 We demonstrate a compact optical fiber microwire current sensor based on the Faraday effect with gigahertz frequency of current sensing capabilities. © 2010 Optical Society of America OCIS codes: 280.4788, 220.4000, 220.4241.
Optical fiber microwires (OFMs) offer a host of enabling properties, such as a large percentage of optical power in evanescent fields, flexibility, configurability, high confinement, robustness, and compactness. These distinctive features have been applied in areas such as telecommunications, sensors, optical manipulation, and high-Q resonators. For sensing purposes, the property of OFMs to guide large evanescent fields has successfully been exploited to devise microfluidic, humidity, refractometric, and biochemical sensors [1–6]. In this Letter, for the first time to the best of our knowledge, OFMs are used for current sensing; the ability of OFMs to guide light, with very low losses over bend radii of the order of a few micrometers has been exploited to minimize the sensor size. This characteristic of OFMs allows for the interrogation of very high frequency (GHz) currents, which has never been previously possible with current sensing schemes using standard optical fibers. In current sensors based on the Faraday effect, the change in the polarization azimuth of the propagating optical field is proportional to the intensity of the magnetic field induced by the applied current. Fiber optic current sensors that exploit the Faraday effect are usually quite bulky [7,8]; this is largely due to the large bend radius needed to achieve lossless optical guidance and to the small value of the Verdet constant of silica necessitating a large number of turns required to produce a detectable current-induced polarization rotation. In certain cases, the problem of sensor bulkiness has been addressed by moving from fiberized to bulk devices and by using different materials where the Verdet constant is relatively higher as compared to that of silica [9–11]. In this Letter, we have exploited the effectiveness of the Faraday effect over a short transit length by devising a very compact optical current sensing system based on OFMs. Figure 1 shows a schematic of the sensing system: light from a laser diode at 1:55 μm is launched into an OFM wound on a copper wire and it is then collected by a detector. Two polarizers are used before and after the sensing head to fix the light polarization status. The 10-cm-long, 5 μm diameter OFM was carefully tapered using the modified flame brushing technique [1] and then wrapped 25 times around a 0:5 mm diameter copper section [Fig. 2(a)], over less than 5 mm length. The magnetic field generated by the passage of current through the small length of copper changes the polarization state of the laser light guided in the OFM wrapped around it. This change in polarization is detected as a drop in in0146-9592/10/183045-03$15.00/0
tensity at the detector, which is otherwise optimized for a certain state of polarization matching that of the input fiber polarizer. Figure 2(a) shows the transmitted optical power from the silica-based OFM before and after packaging: at λ ¼ 1550 nm, the insertion loss was 1:5 dB. Figure 2(b) shows the final packaged sample. Special care was taken while wrapping the OFM around the copper section to minimize the strain-induced birefringence; hence, one can neglect any large effects due to linear birefringence in trying to estimate the rotation in polarization azimuth due to the current flowing in the milled 5 mm copper wire slot. The change in polarization azimuth (dφ) is directly proportional to the interaction length (dLf ) under ~ produced by the curthe influence of magnetic field (B) rent flowing in the copper wire section. This relationship is expressed in Eq. (1) below, where V ðλ;TÞ is the material dependent Verdet constant, which is both dispersive and temperature dependent: Zφ
ZLf dφ ¼
0
~ f: V ðλ;TÞ BdL
ð1Þ
0
On solving Eq. (1), the relationship of the polarization azimuth can also be expressed in terms of the applied current flowing through the milled copper wire as shown below: φ ¼ V ðλ;TÞ μNILf ;
ð2Þ
where N is the number of turns of OFM around the current (I)-carrying conductor and μ is the magnetic permeability of the conductor. To demonstrate the flexibility and effectiveness of the sensor, current flowing
Fig. 1. Layout of optical fiber microwire current sensing system using a single-turn copper frame as the secondary coil. © 2010 Optical Society of America
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OPTICS LETTERS / Vol. 35, No. 18 / September 15, 2010
Fig. 2. (Color online) (a) Final packaged sample used in the current sensing setup. (b) Transmission spectrum of the optical fiber microwire.
through the milled section was subjected to alternating as well as pulsing currents. Alternating current frequencies of 1 and 2 kHz were successfully achieved with the step down (×150) single-turn transformer. To impose higher frequency current signatures to the OFM current sensor, pulsed currents had to be used. This was largely due to the loss of the iron core transformers at high frequencies (>20 kHz). Using pulsed currents also helped to avoid birefringence and changes in the temperaturedependent Verdet constant, which could arise as a result of heating effects associated with continuous flow of high currents. The linearly polarized laser light was set to 45° to the axis of the polarizer. The optical output power, in the absence of signal current, can be expressed as I O ¼ I inp cos2 ð45°Þ:
ð3Þ
Switching the AC signal current leads to angular modulation of the optical signal: I O ¼ I inp cos2 ð45° þ φm sinð2πf tÞÞ:
ð4Þ
Since φm (polarization azimuth) ≪1, the equation simplifies to
Fig. 3. (Color online) Linear relationship between the change in the polarization azimuth with increasing current strength.
Fig. 4. Responsivity over the frequency range, subhertz to 2:0 KHz.
I O ¼ I inp
ð1–2φm sinð2πf tÞÞ ; 2
ð5Þ
where f is the frequency of the input current. With a stable optical intensity (I inp ), the amplitude of the polarization rotation can be ascertained by measuring the amplitude of the first harmonic using a lock-in amplifier. Figure 3 shows a linear relationship between polarization rotation and current in the range of 0 to 19 Amps for alternating current frequencies of 1 and 2 kHz; it has to be stressed that 2 kHz is the highest frequency of continuous currents detected with a fiber optic current sensor exploiting the Faraday effect, exceeding previously reported continuous current values of less than 0:1 kHz [7,8,11]. A current responsivity of 16:8 0:1 μrad=A was demonstrated over alternating current frequencies of subhertz to 2 kHz, as shown in Fig. 4. To calculate the value for the noise-equivalent current, the data acquisition system was set to a 10,000 points running average with a sampling frequency of 50 kHz, yielding a bandwidth of 5 Hz. The personal computer recorded this average every 0:2 s for a period of 20 min, for both with and without signal current (Fig. 5). From Fig. 5, the noise amplitude for no signal current was measured to be 1:56 × 10−6 rad, 5 Hz bandwidth, pffiffiffiffiffiffiyielding a noise-equivalent current value of 0:04 A= Hz, using the measured current responsivity of 16:8 0:1 μrad=A. Using the magnetic permeability for free space, (μ0 ¼ 4π × 10−7 H=m), the Verdet constant for silica (V ¼ 0:54 rad T−1 m−1 ), and the number of microwire turns (N ¼ 25) used for this sample in Eq. (2), the theoretical value for current responsivity was estimated to be 16:9 μrad=A and is in close agreement with the measured value. The sensor in its present configuration of 25 microwire turns has been found to exhibit 16:8 0:1 μrad=A responsivity; even higher responsivities can be envisaged by simply increasing the number of turns, as has been used in other Faraday-effect-based fiber optic sensors [7,8,11]. A 10-cm-long OFM, packaged with great precision on 0:5 mm diameter of milled copper section would yield nearly 60 turns, which would then result in a 60=25 turn proportionate increase in responsivity. To further
September 15, 2010 / Vol. 35, No. 18 / OPTICS LETTERS
Fig. 5. Plot of noise-equivalent current, calculated by observing the noise floor corresponding to no signal current.
increase the responsivity, materials with higher Verdet constant could be used. Still further advances in the fabrication technology of microwires and microcoils is needed, as low-loss microwires and microwire resonators have only been achieved using silica. Higher losses would reduce the benefit of using materials with higher Verdet constants. By exploiting the unique ability of OFMs to guide light under very small bend radii, a reasonable number of turns can be achieved over small dimensions, thus allowing detection of rapid flows of currents. For a 0:5 mm
Fig. 6. (Color online) Temporal response of the optical fiber microwire-based current sensor to a pulsed current. The rise time is limited by the rise time of the current pulse.
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diameter copper section, 25 turns would require an OFM with 4–5 cm of effective length. The effective length governs the transit time of the light through the sensor, and hence the frequency response of 5 cm corresponds to 5 GHz bandwidth. Although a 10 cm long OFM used in this experiment is, in principle, capable of sensing currents in the frequency regime of 2 GHz, it has presently been demonstrated showing rise times of 1–2 μs, as shown in Fig. 6, due to limitations of our pulsing high current system. Figure 6 shows the detector output for ∼2 μs pulses. The pulsed current was produced by a capacitor charged to 15, 20, 25, and 30 V. In conclusion, the proposed microwire current sensor provides the ability to sense high frequency currents or magnetic fields even in hazardous conditions, such as in nuclear environments [12,13], with unrivalled compactness. It has the potential for monitoring electrical breakdowns and coronas, where electric and magnetic field changes typically occur over rise times of ∼1 ns or possibly in applications related to nuclear environments, such as mapping of the electromagnetic fields in fusionreactor environments. References 1. G. Brambilla, J. Opt. 12, 043001 (2010). 2. L. Zhang, F. Gu, J. Lou, X. Yin, and L. Tong, Opt. Express 16, 13349 (2008). 3. F. Gu, L. Zhang, X. Yin, and L. Tong, Nano Lett. 8, 2757 (2008). 4. F. Xu, P. Horak, and G. Brambilla, Opt. Express 15, 7888 (2007). 5. P. Polynkin, A. Polynkin, N. Peyghambarian, and M. Mansuripur, Opt. Lett. 30, 1273 (2005). 6. C.-Y. Chao and L. J. Guo, J. Lightwave Technol. 24, 1395 (2006). 7. A. D. Kersey and D. A. Jackson, J. Lightwave Technol. 4, 640 (1986). 8. R. I. Laming and D. N. Payne, J. Lightwave Technol. 7, 2084 (1989). 9. S. P. Bush and D. A. Jackson, Opt. Lett. 16, 955 (1991). 10. T. Yoshino, K. Minegishi, and M. Nitta, Meas. Sci. Technol. 12, 850 (2001). 11. K. B. Rochford, A. H. Rose, M. N. Deeter, and G. W. Day, Opt. Lett. 19, 1903 (1994). 12. A. F. Fernandez, B. Brichard, F. Berghmans, P. Rodeghiero, A. Hartog, and P. Hughes, IEEE Trans. Nucl. Sci. 52, 2689 (2005). 13. F. Berghmans, A. F. Fernandez, B. Brichard, F. Vos, M. Decréton, A. Gusarov, O. Deparis, P. Mégret, M. Blondel, S. Caron, and A. Morin, Proc. SPIE 3538, 28 (1999).
